The invention concerns a method for inhibition of growth of organisms on faces of constructions submerged in a liquid, in which method an electrically conductive structure to be protected is connected as the cathode of a source of direct current, or an electrically non-conductive structure to be protected is first coated with an electrically conductive material and connected as the cathode of a source of direct current, respectively, and, as the anode, an anode is used that has been isolated from the structure to be protected or that is placed separate from said structure, which anode is connected as the anode of the source of direct current.
The phenomenon of fouling means covering of faces in contact with water with colonies formed by organisms adhering to said faces. Fouling is produced both by micro-organisms and by plants and animals. Fouling usually starts with adhering and spreading of populations of bacteria over faces that are in contact with water. The bacteria pioneers are followed by numerous different algae and other organisms with genuine nuclei, such as barnacles and polyps.
The fouling phenomenon is perhaps most harmful to waterborne traffic (the fuel consumption may increase by up to 40 percent), to industrial plants and power stations that use seawater, and to fish breeding plants.
In the waters of Finland, the fouling trouble was little in the past years. Eutrophication of the water areas near the coasts of the Baltic Sea and an increase in the salt content have increased the disadvantages caused by fouling, in particular in the case of industrial plants that use seawater.
The biggest problems caused by fouling occur in areas in which the salt content in seawater is higher than 5 per mil. In a warm area of seawater which contains salt, fouling is a serious problem for all structures present in the seawater and for all industrial plants and power stations that use seawater as well as for fishing industry. For example, the numerous population in Asia lives mainly on seafood. Ships cannot leave the ports before mechanical cleaning of propellers and other control devices has been carried out.
In order to prevent drawbacks of fouling, at present mainly so-called anti-fouling paint is used. From the anti-fouling paint, one or several substances toxic to the organisms adhering to the structures are separated, such as, for example, copper and tin compounds. In addition to the toxic agents, the smooth face of the paint makes the adhering of the organisms more difficult. However, the anti-fouling paint must be renewed, on the average, at intervals of two years. Organic tin compounds are efficient in combatting the fouling organisms on underwater structures, but they are also toxic for other groups of organisms, such as fish and mammals. Moreover, TBT (tributyl tin) is a poison that accumulates in organisms to a great extent.
Plants and animals can accumulate copper present in dissolved form to a certain extent. Accumulation of copper in the food chain is not known at present, but if high concentrations of dissolved copper are present in water, it may be dangerous to the organisms in the water.
With respect to the prior art, reference is made to the Patent GB-2,118,972, in which the anti-fouling effect described is based on sacrificial Cu/Al or Fe rods. In this prior-art method, Cu/Al or Fe rods are dissolved by means of direct current, and the system of seawater pipes or equivalent that constitutes the structure operates as the cathode. For example, the copper-aluminum hydroxide that is formed prevents formation of growth.
In the method described in the publication EP-0,145,802, the anti-fouling effect is produced by means of sacrificial metal plates, most commonly by means of Cu plates. In this method, the structures to be protected are coated with an insulating layer, onto which a metal plate of a certain size is attached, the size depending on the length of the ship. The protection against corrosion of the structures is effected by supplying a DC-voltage to the hull while graphite, cast iron, platinum-coated titanium, or a Pb/Ag-alloy operates as the anode. The source of DC-voltage consists of a potentiostat, which automatically maintains the potential of the structure to be protected at the pre-set protection potential. The copper hydroxide that is dissolved prevents formation of growth.
In the method described in the publication U.S. Pat. No. 5,009,757, a particular inner Ti electrode and a source of current are employed, and a high capacitance is produced between a zinc coating and the seawater. The zinc-painted hull of the ship operates as the negative terminal of the capacitor. The anti-fouling effect is based on the Helmholtz double layer produced by the electric current between the zinc coating and the seawater.
The anti-fouling effect described in the Pat. Appl. FI-915300 is based on ultrasound. The low-frequency oscillations of the sources of ultrasound make the micro-organisms to be separated from the face of the structure.
In the publication EP-0,468,739, a direct-current method is described, in which an electric shock is given to the microbes growing on the faces to be protected by means of an electric field produced between separate electrodes. In this method, the structure to be protected is not connected to the source of current, but the electric current is passed through a separate displaceable anode to a separate displaceable cathode.
In the publication EP-0,369,557, a direct-current method is described, in which the structure to be protected is coated with a conductive layer, on whose face, in an electrolysis of seawater, an anode reaction forms hypochlorite which kills microbes.
In the publication WO-87/03261, a method based on the use of alternating current is described. In this method, the organisms are destroyed by means of an electric shock produced by means of the field of alternating current. The effect can be intensified by dissolving copper, aluminum and by electrolyzing seawater by means of direct current, in which connection the chlorine gas that is formed kills microbes.
The prior-art methods involve a number of drawbacks. When anti-fouling paints are used, damage to the environment constitutes the major drawback. Also, the annual cost of maintenance becomes relatively high. Moreover, the anodes that are consumed on dissolution of copper, aluminum and iron cause a need of maintenance.
In the ultrasound method, the most important drawbacks are the high cost of the method and the detrimental effects of resonance.
The prior-art electrical methods also involve a number of significant drawbacks. In cases in which the object to be protected is subjected to an external electric field (direct or alternating current), separate electrodes that supply current are needed. Also, in these prior-art methods, a control system that optimizes the current is missing. An excessively high current density produces the risk of hydrogen brittleness in electrically conductive structures. Oxidation, i.e. wear, of a paint that operates as an anode is a clear drawback.
In a method that makes use of the Helmholtz double layer, precipitation of calcium and magnesium on the face and, consequently, formation of a face favourable for growth, is the most important drawback.